Re: [ATM] Mirror cell design(silicone)

[James P Crombie <jpcrombie@pei.eastlink.ca>]

Why its the mirror making leprecons :-)  They magically figure mirrors 
while you sleep :-)

Richard Schwartz wrote:

> Where did they get those tiny little men only 1" tall who were walking
> around on the mirror blank?
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>> [Original Message]
>> From: James P Crombie <jpcrombie@pei.eastlink.ca>
>> To: <atm@atmlist.net>
>> Date: 7/21/2004 3:41:09 PM
>> Subject: Re: [ATM] Mirror cell design(silicone)
>>
>> Mark
>>  For some more info on silicone on glass, see this article from the 
>> Steward mirror lab and their problems with silicone.
>> http://medusa.as.arizona.edu/mlab/lbtglue2.html
>>
>> Mark Holm wrote:
>>
>>   
>>
>>> Just in case you did not notice, Richard Schwartz rather enjoys 
>>> writing the opposite of what he means, giving generous clues that he 
>>> is doing so by including outrageous statements.  It is a little 
>>> oddity of his personality.  Once you are used to it, you find that 
>>> he usually has useful things to say.  Richard's style could be a bit 
>>> confusing for someone encountering it for the first time, especially 
>>> someone for whom English is a second language.
>>>
>>> In fact, a lot of telescope mirrors have been successfully mounted 
>>> with silicone adhesive.  The big advantage is that it frees one from 
>>> having to support the edge.  The big disadvantage is we don't have a 
>>> good tool yet for quantitatively predicting whether one will get 
>>> trouble when the cell is not horizontal.  Enough people, including 
>>> professional opticians and astronomers, use this method that it can 
>>> be recommended as probably no more likely to cause trouble than 
>>> other methods of edge support.
>>>
>>> It is certainly possible, and perhaps even prudent, to build some 
>>> mechanical edge and front restraints into your cell as well as 
>>> trusting the silicone.  Make these not quite touch the mirror in 
>>> normal use.  Their purpose is to provide a failsafe in case the 
>>> silicone should let loose.
>>>
>>> The best silicone easily available is probably clear aquarium 
>>> sealant or, perhaps in second place, clear bathtub caulk.  You want 
>>> clear, not so much because pigment might be harmful, but because 
>>> pigment might hide the dilution of silicone with cheaper polymers.  
>>> That would probably make the product less adhesive to glass.  It is 
>>> hard to make the sealant or caulk reasonably transparent unless it 
>>> is fairly pure silicone.  Good silicone sealant or caulk will be 
>>> more expensive than other sealants or caulks.  In the US, there are 
>>> some brands that mix silicone with cheaper polymers.  Read the 
>>> labels carefully to avoid these products.  Ask here on the list if 
>>> in doubt.
>>>
>>> Also, you want fresh silicone.  Don't use a tube of caulk you have 
>>> left over from a remodeling project five years ago.  Buy new, and 
>>> try to buy from a store where the stock looks fresh.  If the 
>>> container has a thick layer of dust on it, look for another source.  
>>> Old silicone sometimes loses it's ability to cure properly.
>>>
>>> Regarding the back support:  1. Do not use the proportions listed in 
>>> Kriege and Berry.  Instead, send your mirror dimensions to this 
>>> list, and someone will use Plop to calculate proportions for you. 
>>> (or you can download and use Plop yourself.  Google on Lewis Plop.)  
>>> We need: glass type, mirror diameter, mirror thickness (at the 
>>> edge), mirror focal length, planned diagonal mirror diameter (an 
>>> estimate is good enough for the diagonal mirror diameter).  2. If 
>>> you are going to try gluing the mirror to the supports with 
>>> silicone, use thicker screws than K & B recommend.  The screws will 
>>> be supporting the weight of the mirror in bending as well as 
>>> compression and you will want the extra stiffness.  Also, you will 
>>> want to eliminate as much sideways play as possible from the mechanism.
>>>
>>> Mark Holm
>>> mdholm@telerama.com
>>>
>>> _______________________________________________
>>> ATM mailing list http://www.atmlist.net/
>>>
>>>     
>>
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